During sulfite cooking about half of the solid wood substance (on the average) is chemically transformed by sulfonation, hydrolysis and oxidation to water soluble products, which are contained in the waste liquor. More than half of this dissolved organic wood substance is made up of lignin in the form of lignosulfonates. The lignin present in the waste liquors is therefore not the original lignin of the wood but a lignin derivative having properties which require a further chemical transformation in order to be useful.
Lignin, like phenol, easily form condensation products with carbonyl compounds such as formaldehyde. Unlike the phenols, the lignin, on account of its high degree of polymerization, contains fewer phenolic hydroxyl-groups and thereby fewer active sites on the aromatic ring for linking with e.g. formaldehyde. If unmodified lignosulfonate is condensed with formaldehyde a three-dimensional network (a resin) will certainly be obtained but it will have a limited number of linking points and therefore will be more brittle and weaker than a phenol formaldehyde resin. Furthermore, sulfonic acid groups will enhance the water swellability of the network, which means an impairment of the water resistance of the bonded product.
There have been a number of efforts by prior art workers to improve the properties of condensation products of formaldehyde and lignosulfonates. The object of our invention is to provide an improved process over what has gone before.